Waymo says it's working on improving cars' foul-weather capability.

Prototype Waymo self-driving cars have been picking up passengers in Arizona since December. But the cars may still have some technological hurdles to overcome, according to a Waymo passenger interviewed by The Verge, such as driving in the rain.

Shawn Metz is a 30-year-old human resources manager living in Chandler, Arizona. He's one of the people selected by Waymo to use the Google spinoff's ride-hailing service, called Waymo One. The service allows users to hail a Waymo vehicle with an app, similar to Uber or Lyft. Metz has been more vocal about his experiences than other users, however.

Among the many topics discussed in a lengthy interview, Metz said he hailed a Waymo car during a recent rainstorm. When he got in, he said the car was being driven manually (Waymo puts human safety drivers in all of its cars as a backup). It's an example of the many challenges autonomous-driving tech still faces, even as companies make ambitious claims about commercialization. One of the advantages of operating in Arizona is that weather isn't much of an issue most of the time. Waymo said it does operate cars in light rain, and that it is working on improving cars' foul-weather capability by testing in other locations.

"We've tested in over 25 cities across the U.S., which allows us to learn in different kinds of environments and weather conditions," a Waymo statement said. "So far, we've taught our cars to drive in moderate weather (rain) and are gaining more experience in Kirkland, Washington. To test in snow, we have cars driving in Novi, Michigan, during winter months. And in San Francisco, we're building up experience driving in fog. Investing in weather testing is crucial to being able to bring our technology to more people in more places."

Metz noted a second incident in which a Waymo car was flummoxed by a crowded Costco parking lot. After failing to find a path through pedestrians walking in and out of the store, the car "timed out," necessitating a call to Waymo's remote support center for help, Metz told The Verge.

Overall, Metz was fairly positive about Waymo's self-driving cars. He said he didn't blame Waymo for being cautious and having a human driver take over during the rainstorm. He also had no sympathy for area drivers who have expressed frustration at the Waymo cars' conservative driving style, and strict adherence to speed limits. Metz said he has not experienced any of the belligerence against Waymo vehicles described in recent reports, which included a person pointing a gun at one of the cars.

Metz said he and his wife use Waymo One six to eight times a week. When not using Waymo One, Metz primarily relies on a bicycle. He had never used Uber or Lyft prior to signing up for the Waymo service, according to The Verge.